Opiate peptides and opiate receptors are concentrated in parts of the limbic system and striatal complex. It has been suggested that endogenous opiates may be involved in sexual behavior. This possibility was tested by observing the effects of the opiate antagonist naltrexone on the mating behavior of male hamsters. A detailed ethological analysis revealed several aberrations in the mating behavior of naltrexone-treated animals. In general, their behavior appeared sluggish and inattentive. They were capable, however, of successfully impregnating females.